1. Field of the Invention
The present invention relates to a method and apparatus for handling random access procedure in a wireless communications system, and more particularly, to a method and apparatus for implementing timing alignment and resource request, so as to enhance system efficiency.
2. Description of the Prior Art
The third generation mobile telecommunications system (called 3G system) provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission, and also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates. However, due to demand for high-speed and multimedia applications, the next generation mobile telecommunications technology and related communication protocols have been developed.
In the Long Term Evolution wireless communications system (LTE system), a Random Access Channel (RACH) is configured in an uplink (UL) channel between a user equipment (UE) and the network, and utilized for timing alignment, RNTI (Radio Network Temporary Identifier) assignment, and resource request. In the starting state, only downlink (DL) timing between UE and Node B (NB) is synchronized, and signals on RACH can be used for UL timing alignment. Before performing UL timing alignment, UE uses downlink Synchronization Channel or reference signals to perform synchronization on timing and frame. However, since signals may be delayed due to a distance between the transmitter and receiver, UE is not able to determine whether a message transmitted from UE is at a starting position of a receiving frame of NB. In addition, when NB provides service for multiple UEs at the same time, Round Trip Delays (RTDs) between each UE and NB may be different due to different distances thereof, causing timing offset. Therefore, NB evaluates timing offset of a UE according to RACH signals from the UE, and notifies the UE to adjusting UL timing via Downlink Shared Channel (DL-SCH), to achieve timing synchronization. A RACH signal is composed of preambles used for UL timing synchronization and UE identity detection, or carrying short signalling or signature.
On the other hand, RACH signals among different UEs are transmitted to NB by contention-based or non-contention-based method. That is, UE can select a RACH transmission opportunity and randomly select a preamble to transmit signals, or transmit a signal carrying a preamble assigned by the network (in such a situation, the RACH transmission opportunity is assigned by the network or selected by the UE). If the network cannot identify the transmitted RACH signal due to collision or low power, or if contention lost occurs due to a randomly-selected preamble, the UE can retransmit the RACH signal with larger power in the next available RACH transmission opportunity, until a response from the network is received or a condition is reached, e.g., maximum transmitting or maximum power.
For LTE, a RACH transmission opportunity is related to a time-frequency radio resource, not only related to time or frequency. Therefore, when RACH physical resource is selected, its time period and frequency band are determined. Of course, at certain time period, there may be more than one frequency sub-band for selection. On the other hand, preamble can be random access preamble, which is randomly selected by UE, or dedicated preamble, which is assigned by network (source cell or target cell). Basically, when UE uses (randomly selected) random access preamble, there is possibility another UE use the same preamble and transmit it in the same RACH opportunity so that network doesn't know whether the signal comes from one or more than one UE. Consequently, contention exists and will be solved after UE sends its UE identity in subsequent message 3. In contrary, dedicated preamble is assigned by network to a known specific UE so it's unique and won't cause contention between UEs (no another UE use the same dedicated preamble.)
In LTE, asynchronous RACH is concluded and adopted as working assumption where dedicated preamble and random access preamble based RACH accesses may both be supported. Random access procedure performance mainly in terms of latency and overhead is affected by collision/contention probability, time/frequency resources, number of user equipments (load), number of preamble signature, channel quality, UE identities, and even access causes and so on. On the other hand, the design requirements in addition to performance, such as short life span of identity usage, to be common for various kinds of non-synchronous RACH accesses in E-UTRAN (eNB and aGW) for FDD and TDD as well as irrespective of cell size, optimization for connected state UEs, are considered while possible reasons for UE to access on RACH can be categorized into four main causes, which are initial access (e.g. UE originated call, network originated call, tracking area update and initial cell access) including initial NAS signaling for NAS procedures (e.g. service request, network attach, routing/tracking area update), synchronization request, handover access and scheduling request. According to requirements and need of support for possible access causes, the baseline random access procedure model and channel mapping (between logical channels and transport channels) are defined in the prior art. Current decision on four steps of random access procedures is shown as following:
(1) Step “Random Access Preamble on RACH in uplink”: is corresponding to a message 1, which carries 6 bits, and indicates a random ID, and possibly other information, e.g., cause or size, potentially with priority, pathloss or CQI to allocate UL resource appropriately.
(2) Step “Random Access Response on DL-SCH”: is corresponding to a message 2, which is semi-synchronous (within a flexible window of which the size is one or more TTI) with message 1, no HARQ (Hybrid Automatic Repeat Request) support, transmitted on L1/L2+DL-SCH, addressed to RA-RNTI (Random Access RNTI) on L1/L2 control channel, conveys at least RA-preamble identifier, timing alignment information, initial UL grant and assignment of Temporary Cell RNTI (T-CRNTI), and is intended for one or multiple UEs in one DL-SCH message.
(3) Step “First scheduled UL transmission on UL-SCH”: is corresponding to a message 3, which uses HARQ, operates in RLC TM (Radio Link Control Transparent Mode) without segmentation, conveys at least UE identifier and (explicit or implicit) information on whether C-RNTI is already available. Besides, in case of initial access and if the size of the message allows it, the initial NAS message (or something allowing to build the initial NAS message in eNB) can be included, and size of the message is dynamic.
(4) Step “Contention Resolution on DL-SCH”: is corresponding to a message 4, which is not synchronized with message 3, and addressed to the Temporary C-RNTI on L1/L2 control channel (at least for initial access). Besides, content of the message is FFS (for further study), HARQ is supported, and HARQ feedback is transmitted only by the UE which detects its own UE identity, as provided in message 3, echoed in the RRC (Radio Resource Control) Contention Resolution message.
In the prior art, at initial access, the four steps are: Random Access Preamble on RACH, Random Access Response via CCCH (Common Control Channel) on DL-SCH, RRC Connection Request via CCCH on UL-SCH, and RRC Contention Resolution via DCCH on DL-SCH.
Therefore, for LTE, the prior art provides steps of RA procedure. However, some problems may occur.
First, during random access procedure, radio link failure, handover (e.g. changing new serving cell), and tracking area update can happen. It's possible that after an UE informs eNB (enhanced Node B) in message 3 about its already having C-RNTI (e.g. by index or provide its C-RNTI directly), the C-RNTI held by the UE may either have been released or become invalid before message 4 (e.g. message 4 is asynchronous to message 3 and three retransmissions are allowed) addressed by UE identity (e.g. can be invalid as well) is received by the UE. Consequently, for example, network may consider that the UE uses its original C-RNTI so as to relocate T-CRNTI assigned in message 2 to other random access UEs while the UE adopts the previous received T-CRNTI as its C-RNTI. In addition, according to unique (or unique in most cases) UE identity provided in message 3, network shall know whether the UE has C-RNTI already. It's unnecessary to provide notification of having C-RNTI to result in unnecessary overhead. Moreover, for some causes of initiation of random access (e.g. scheduling request) during which some scenarios could happen (e.g. synchronization request or tracking area update), the purpose of initiating RA may become not essential and critical after completion of random access procedure.
In UMTS, downlink resources are allocated by CRNC when HS-SCCH sets are configured/decided by NB. However, in LTE, there is no CRNC any more. It's not clear how network maintains the resource allocation and configuration. In addition, it's necessary to consider how continuous packet connectivity feature is fulfilled in LTE at handover.
In LTE, after a UE initiates random access procedure by sending a preamble (random access preamble or dedicated preamble), the UE should expect the random access response message for both contention-based and non-contention-based random access procedures from network if the network receives the preamble over RACH. In addition, the random access response message shall be addressed to a RA-RNTI corresponding to or uniquely identifying the accessing RACH time-frequency resource where the RACH accessing pattern (time-frequency within radio frame) is indicated. According to the procedure, network shall send the random access response message within flexible window (e.g. one or more TTI). However, if the network doesn't send random access response message earlier enough so that scheduled the same RACH time-frequency resource (identifying by the same RA-RNTI expected) is reached before reception of the response message (e.g. random access response message is received later than scheduled time slot of next period of the same RACH time-frequency resource within a radio frame or expected reception time for accesses at next period of the same RACH time-frequency resource within a radio frame), the problem will happen.
For example, UEs sending preambles at next period of the same RACH time-frequency resource expect the same associated RA-RNTI by wasting power in reception response message which actually is intended for UEs with access attempts at previous period of the same RACH time-frequency resource. Consequently, the power control mechanism for access attempts at next period of the same RACH time-frequency resource cannot work correctly to reflect the UL channel conditions between base station and UEs with access attempts at next period of the same RACH time-frequency resource so as to may further impact subsequent transmissions. Therefore, for contention based case, UEs with the same used random access preambles at next period of the same accessing opportunity may also consider the response message intended to the UEs with the same random access preambles sending at previous period of the same accessing opportunity is intended to them so that they will all send message 3 in UL where the network cannot really differentiate which UE accesses the RACH time-frequency resource in the previous period of the accessing pattern and which one accesses later. Therefore, it's unfair to the UE sending access attempts earlier.
On the other hand, for non-contention based cases, if the network doesn't carefully consider end time of a dedicated preamble for a UE with the response window (e.g. end time expires earlier than the instant of transmission of random access response message or expires earlier than the instant of reception of random access response message), the UE behaviour is unspecified and there may be two UEs receive the same timing alignment information which is especially serious to the non-contention based case since it doesn't have contention resolution message to resolve the contention problem. In addition, for contention based case, the delay on completion of random access event, UL interference and unnecessary power consumption will be increased.
Since in random access procedure message 4 is not synchronized to message 3, once a UE doesn't detect control channel information (addressing to it) or DTX/ACK happens, the UE may be delay (e.g. until finding T-CRNTI which is reassigned to others) or even wait for forever (e.g. network think T-CRNTI is adopted because of DTX/ACK so that won't assign to anyone else) if no specific action is specified.
In addition, if C-RNTI is used to address message 4 for RRC connected UE, after a UE sends message 3, the UE may wait for long time (e.g. until its assigned T-CRNTI is reused by a winner UE) or even forever (e.g. no winner UE uses the T-CRNTI) since message 4 is not synchronized to message 3.
When the C-RNTI of a UE is detected or known by network upon receive message 3, it's unnecessary to address T-CRNTI on L1/L2 control channel for winner UE since UE knows its own C-RNTI if there is one. The life span of T-CRNTI should be able to be terminated before estimated duration of reception of T-CRNTI is reached. Otherwise, the T-CRNTI may be out of stocks/in shortage or availability of T-CRNTI to a UE may be delayed.
For some random access causes (e.g. handover or synchronization request), network may assign dedicated preambles to some UEs going to initiate random access triggered by these causes to avoid (if there is enough RACH time/frequency resources) or reduce contention (if too many UEs asking for accesses). Since normally an UE receiving dedicated preamble is in connected state and its context is available at network entity beforehand, the network entity shall know whether the UE has C-RNTI or not beforehand. It seems that it's unnecessary to assign T-CRNTI in message 2 in this kind of scenario. On the other hand, even T-CRNTI may be considered necessarily to be issued, it's unnecessary to send whole T-CRNTI on DL-SCH which not only consumes radio resource but also limits the number of UEs which can be dealt with during random access procedure in message 2 (e.g. message size so that the number of UEs being able to receive message 2 is limited if we assume each UE requires certain amount of information length in message 2).
HARQ is supported for contention resolution message with allowing one retransmission. However, acknowledgment errors can lead to confusion between network and UEs. If DTX/ACK or NACK/ACK happens, for UEs originally without C-RNTI, the network will consider the UE adopts T-CRNTI as C-RNTI while the T-CRNTI is not actually used. In contrast, if ACK/NACK happens, the network will consider the UE doesn't adopt T-CRNTI as C-RNTI while the T-CRNTI is actually occupied. The further problem may occur when two UEs consider they have same C-RNTI.
In random access procedure, message 4 is supported by HARQ. Therefore, if DTX/ACK happens, the network may consider an assigned T-CRNTI is adopted as a UE's C-RNTI while it's released.
For both contention based and non-contention based random access procedure, no matter whether HARQ is supported for random access response message or not, a UE waiting for the response message should not send NACK to network corresponding to the response message. Otherwise, if acknowledgement errors happen (e.g. ACK/NACK or NACK/ACK), following UL-SCH transmission will result in radio disturbance and may be miss detected. In addition, the UE identity may be occupied at the UE but the network considers the UE identity is free to assign. Moreover, the dedicated preamble may still be used by the UE when the network assigns the same dedicated preamble to another UE.
NAS procedures, such as tracking area update (TAU), network attach (NA), and service request (SR), can be initiated when the UE is in LTE_IDLE or LTE_DETACHED. During these procedures, it's possible for NAS layer to initiate re-authentication even though it's relatively rare for cases of TAU and SR procedures. In addition, duration for completion of either TAU or SR procedure usually is quite short (note: for NA, duration of procedure might be quite long) so that it would be rare that a handover would be required within the duration. On the other hand, during initial NAS signaling such as NA, handover may need to be supported before S1 context has been available at eNB and RRC security has been established, shown as FIG. 2. However, some impacts may be expected (e.g. incomplete RRC context at eNB from S1, potential security attacks and denial of service request, necessity of UE capability sent over radio interface, necessity of forwarding of S1 and NAS signalling over X2 interface). The issue should be solved.